Thermal transfer printing is a method wherein a thermal ink film is heaped on an image receive sheet and heated by a thermal head to print images directly onto a receive sheet. When a sublimable dye is employed in this method, it is known to the art that the obtained image properties are very good like photographs. The photograph like image, however, is not obtained when the receive sheet is plain paper, because the plain paper has rough surface and it is difficult to fix the image on it. In order to obtain the photograph-like image, it is necessary that particular paper sheets have a printing layer onto which the sublimable dye is easily fixed. It is, however, desired to form the photographical image on plain paper.
In order to satisfy this desire, it is proposed that the images are preliminarily transferred on an intermediate sheet having a printing layer and then only the printing layer is transferred onto a receive sheet (see U.S. Pat. No. 4,923,848). In this process, the intermediate sheet and the thermal ink film are sandwiched between the thermal head and a platen roller under a certain pressure, and thermal printing is conducted. Among the thermal transfer printing, the method employing the sublimable dye requires energy several times larger than the conventional hot melt type thermal transfer printing process. It is therefore required that the printing layer on the intermediate sheet be anchored on the substrate of the intermediate sheet even after such higher energy printing. Since the sublimable thermal transfer printing is generally applied for full color printing, the heating step with the thermal head should be conducted at least three times, after which the printing layer is required to be anchored on the substrate of the intermediate sheet. Contrary to this step, the printing layer is adhered onto the receive sheet by heat or pressure and then the substrate of the intermediate sheet is necessary to be peeled off in the next step. It is therefore required that the printing layer of the intermediate sheet have two properties which are in conflict with each other. Especially in the sublimable dye, if the color layer of the ink film and the printing layer have high heat resistance, printing sensitivity significantly lowers. Both layers should be prepared from a material having lower heat resistance, and therefore easily gives rise to problems of heat fusion between the printing layer and the color layer of the ink film or between the printing layer and the substrate of the intermediate sheet.
It is proposed that the printing layer is prepared from saturated polyester resin. However, since the substrate to be covered with the printing layer is generally formed from polyester, the adhesion power between the polyester printing layer and the polyester substrate is quite strong and therefore difficult to peel the substrate off after attaching the printing layer onto the receive sheet. It is also considered that a releasing layer is disposed between the printing layer and the substrate. The releasing layer in turn allows the printing layer to transfer onto the thermal ink film during heat printing with the thermal head.
In order to promote the adherence of the printing layer onto the receive sheet or to inhibit tranferring the printing layer onto the color layer of the ink film, it is proposed that an adhesive layer is disposed either between the printing layer and the substrate of the intermediate layer or on the surface of the printing layer. Since the adhesive layer is thermoplastic at ambient temperature, the printed images in the printing layer often bleed into the adhesive layer. The adhesive layer also has adhesive properties to everything and may give rise to mechanical operation and treatment problems.